Chapter 9. Camshafts and Valve Events.|
12919 words. 44 illustrations.
Historical Background. Post war performance demands. 10,000 r.p.m. with pushrods. 20,000 r.p.m. without - eventually.
Mechanisms. Sleeve valves. High powered aircraft engines. Two stroke valve types. Rotary valves. The poppet valve. Basic lift curve. Direct action, pushrods, rockers, etc. Practical matters.
Valve Events. Valve events diagram. Effect of the various opening and closing events.
Valve Events and Piston Motion. Interaction of valve events, piston motion and cylinder pressure. Phasing of piston velocity with valve lift. Defining the valve opening period.
Evolution of Lift, Duration and Timing. Effect of engine size. Rolls-Royce Merlin. Road car engines prior to the 1950s.
The Ford Flat Head V8. More advanced than it might look. Early use of complex cam forms.
Post WW2 Progress. BMC A Series. Manx Norton. Desmodromic solutions. Mercedes F1 V10. Jaguar V8 VVT.
Valve Train Dynamics. Side valve simplicity. Chains, gears, belts. Pushrod complexities. 1994 Mercedes-Benz pushrod Indy race engine.
The Cam Base Circle. Influence on minimum cam duration limit. Modern trend to large base circle diameters.
Direct Acting Methods. Bucket tappet. Rocker arms. Lightweight finger follower. Pneumatic springs. Very hard coatings.
Lift, Velocity, Acceleration, Jerk. The cam profile. Shocks from flat flanks. Violent changes of velocity and acceleration. Adding curves to improve. Quietening ramps. 3 arc cam. Constant acceleration cam. Acceleration by curves. Measuring a cam, need for high precision. How lift, velocity, acceleration and jerk relate to cam and tappet motion. Importance of component dynamics. Reshaping the acceleration curve for available components.
Torsional Problems. Torsional forces acting on the valve mechanism. Firing impulses from the crankshaft. Inertia loads from the camshaft. Conflict in between. V8 tribulations. Compliance or damping. Compliant compound gear of Cosworth DFV. Pendulus absorber.
Manufacturing Precision. Traditional copy-grinders. Modern CNC machines. Standards of accuracy and surface finish.
Practical Considerations. Trying to find a balance between precision and flexure that varies with load.